1. Field of the Invention
The invention relates to a display device including an active device, and more particularly to an active matrix type display device including a spontaneous light-emitting device such as an organic electroluminescence (EL).
2. Description of the Related Art
A portable communication terminal such as a cellular phone has been widely used recently. As a display unit of such a portable communication terminal, a liquid crystal display device is widely used.
A liquid crystal display device including a back light unit is accompanied with a problem of much power consumption for enhancing a brightness in a display screen. To solve this problem, a display device including an organic electroluminescence (hereinafter, referred to simply as xe2x80x9corganic EL display devicexe2x80x9d) attracts an attention as a display device suitable to a portable communication terminal, as having been suggested in Nikkei Electronics, No. 765, Mar. 13, 2000, pp. 55-62.
Hereinbelow is set forth a summary of Nikkei Electronics, No. 765, Mar. 13, 2000, pp. 55-62.
As a display device including a spontaneous light-emitting device which emits a light when a current runs therethrough, there are known a plasma display (PDP) device and an electroluminescence (EL) device. An electroluminescence device is grouped into an inorganic one and an organic one with respect to a material of which an electroluminescence device is composed, and is further grouped into a simple matrix type device and an active matrix type device with respect to a structure.
FIG. 1 is a block diagram of a simple matrix type organic EL display device.
As illustrated in FIG. 1, the simple matrix type organic EL display device includes a data line driver circuit 55 to which a plurality of data lines 53 are electrically connected, a scanning line driver circuit 56 to which a plurality of scanning lines 54 are electrically connected, and a plurality of pixels arranged in a matrix.
Each of the pixels is comprised of an electroluminescence device 51, a capacitor 52 electrically connected between an anode and a cathode of the electroluminescence device 51, one of the data lines 53 to which the anode of the electroluminescence device 51 is electrically connected, and one of the scanning lines 54 to which the cathode of the electroluminescence device 51 is electrically connected.
The data line driver circuit 55 activates one of the data lines 53 and the scanning line driver circuit 56 activates one of the scanning lines 54 to thereby supply the electroluminescence device 51 electrically connected to the thus activated data and scanning lines 53 and 54, with a current from the activated data line 53 towards the activated scanning line 54. As a result, the electroluminescence device 51 emits a light with a brightness determined in accordance with the current running through the electroluminescence device 51.
Since a simple matrix type organic EL display device has a relatively simple structure as mentioned above, it can be fabricated with low costs. However, it is difficult for a simple matrix type organic EL display device to increase the number of pixels for accomplishing a higher density in pixels.
Since a scanning line is selected one by one, and then, a light-emitting diode in an associated pixel is made to emit a light in a simple matrix type organic EL display device, a period of time during which a light-emitting diode in a pixel emits a light is equal to A/B wherein A indicates a frame period and B indicates the number of scanning lines. In order to keep a brightness constant in such a limited period of time, it would be necessary to instantaneously flow a much current through a pixel.
If the number of pixels is increased, the data line 53 would have an increased wire length. The data lines 53 are generally composed of a transparent material such as ITO (Indium Tin Oxide), and hence, has a high wire resistivity. As a result, as the data lines 53 have an increased wire length, the data lines 53 would have an increased wire resistance.
Thus, there occurs a significant voltage drop in the data lines 53, because the data lines 53 have an increased wire resistance, and further because a much current runs through the data lines 53.
Such a significant voltage drop results in that a voltage on the data line 53 located farther away from the data line driver circuit 55 becomes smaller than a voltage on the data line 53 located closer to the data line driver circuit 55. This causes that a smaller current runs through the electroluminescence device 51 electrically connected to the data line 53 located farther away from the data line driver circuit 55.
That is, since a smaller current runs through the electroluminescence device 51 electrically connected to the data line 53 located farther away from the data line driver circuit 55, because of an increased wire resistance of the data lines 53, the electroluminescence device 51 would emit a light in a smaller amount, resulting in non-uniformity in a brightness in a display screen. Specifically, a pixel located farther away from the data line driver circuit 55 would have a smaller brightness.
FIG. 2 is a block diagram of a conventional active matrix type organic electroluminescence display device.
As illustrated in FIG. 2, the conventional active matrix type organic EL display device includes a data line driver circuit 68 to which a plurality of data lines 65 are electrically connected, a scanning line driver circuit 69 to which a plurality of scanning lines 66 are electrically connected, a bias voltage source 610, a common bias voltage line 611 through which a bias voltage is applied from the bias voltage source 610, a plurality of bias voltage lines 67 electrically connected to the bias voltage line 611, and a plurality of pixels arranged in a matrix.
Each of the pixels is comprised of an electroluminescence device 61, a first thin film transistor (TFT) 62 electrically connected between an anode of the electroluminescence device 61 and one of the bias voltage lines 67, a second thin film transistor (TFT) 63 electrically connected between one of the data lines 65 and a gate of the first thin film transistor 62, and a capacitor 64 electrically connected between a gate of the first thin film transistor 62 and one of the bias voltage lines 67.
When the scanning line driver circuit 69 activates one of the scanning lines 66, the second thin film transistor 63 electrically connected to the thus activated scanning line 66 is turned on, and hence, a current runs to the capacitor 64 through the data line 65 and the second thin film transistor 63 from the data line driver circuit 68, resulting in that the capacitor 64 is electrically charged.
Thus, a gate voltage of the first thin film transistor 62 becomes high. When the gate voltage of the first thin film transistor 62 becomes higher than a threshold voltage, the first thin film transistor 62 is turned on, resulting in that a current is supplied to the electroluminescence device 61 through the common bias voltage line 611 and the bias voltage line 67 from the bias voltage source 610. Thus, the electroluminescence device 610 emits a light at a brightness in accordance with the current supplied thereto.
As is obvious in view of the above, the active matrix type organic EL display device and is characterized in that even if the number of scanning lines were increased, it is ensured to have a period of time during which a light is emitted, equal to a frame period of time, differently from the simple matrix type organic EL display device.
Herein, an active matrix type liquid crystal display device is compared to the above-mentioned active matrix type organic EL display device.
In an active matrix type liquid crystal display device, a transmissivity, which corresponds to a brightness in an active matrix type organic EL display device, is in proportion to a voltage to be applied to liquid crystal. In contrast, a brightness in an active matrix type organic EL display device is in proportion to a current, and a voltage supplied to the bias voltage lines 67 from the bias voltage source 610 is fixed at a constant voltage.
Since an organic EL display device is driven by a current, a thin film transistor simply conducting an on/off operation cannot be used in an organic EL display device unlike an active matrix type liquid crystal display device. An organic EL display device has to use a thin film transistor having an on-resistance small enough for a current to run therethrough.
Such a thin film transistor having a small on-resistance cannot be fabricated by a process for fabricating an amorphous silicon thin film transistor, and hence, has to be fabricated by a process for fabricating a low-temperature polysilicon thin film transistor which process is usually used for fabricating a display device capable of displaying images with high accuracy.
In a low-temperature polysilicon thin film transistor, a thin film transistor and a driver circuit can be fabricated on a glass substrate. When multi gradation display is to be accomplished, almost all circuits associated with scanning lines and a part of circuits (selection switches) associated with data lines are fabricated on a glass substrate, and a complex circuit for controlling gradation is comprised of a semiconductor integrated circuit formed on a singly crystal substrate.
An active matrix type liquid crystal display device uses red, green and blue color filters for displaying colored images.
In contrast, an active matrix type organic EL display device uses organic EL devices emitting red, green and blue lights, for displaying colored images.
However, the active matrix type organic EL display device is accompanied with problems that an organic EL device emitting a red light has a shorter lifetime than those of organic EL devices emitting green and blue lights, and that the organic EL device does not emit a pure red light, but emit an orange light.
In the active matrix type organic EL display device, red, green and blue lights may be mixed to one another to thereby produce a white light, and pixels associated with red, green and blue may be fabricated through the use of color filters like a liquid crystal display device.
In the above-mentioned simple matrix type organic EL display device, as the number of pixels is increased, a data line would have a longer wire length, and hence, have a greater wire resistance.
Thus, a voltage drop would occur in a data line because of an increase in a wire resistance thereof and further because of a much current running through a data line. This causes a problem that since a current running through an electroluminescence device electrically connected to a data line located remote from the data line driver circuit is reduced, the electroluminescence device would emit a light in a smaller amount, resulting in non-uniformity in a brightness in a display screen.
On the other hand, though the active matrix type organic EL display device has a merit that a period of time during which the display device can emit a light which period is equal to a frame period of time can be ensured, the active matrix type organic EL display device is accompanied with the following problem like the above-mentioned simple matrix type organic EL display device.
Bias voltage lines or transparent electrodes would have an increased wire resistivity and an increased wire resistance, as the number of pixels is increased. This results in that the bias voltage lines would have an increased wire resistance, a pixel located far away from the bias voltage source would have a reduced brightness, and hence, non-uniformity occurs in a display screen of the active matrix type organic EL display device.
A further problem common to the conventional simple matrix type organic EL display device and the conventional active matrix type organic EL display device is that extra power has to be supplied to the bias voltage lines from the bias voltage source in order to compensate for reduction in a brightness in a pixel which reduction is caused by an increase in a wire resistance in the bias voltage lines. This problem is quite serious to a display device required to accomplish reduction in power consumption.
Japanese Unexamined Patent Publication No. 7-326311 has suggested an electron source including M wires extending in a row direction, formed on an electrically insulating substrate, N wires extending in a column direction, formed on said row-direction wires with an insulating layer sandwiched therebetween, and a surface conductive type electron emitting device including a thin film having at least one pair of electrodes and an electron emitter. Each of the electrodes is electrically connected to both the row-direction wires and the column-direction wires. A plurality of the surface conductive type electron emitting devices are arranged in a matrix. The row- and column-direction wires are designed to include terminals through which a voltage is applied thereto, at opposite ends.
Japanese Unexamined Patent Publication No. 10-112391 has suggested a X-Y matrix type organic thin film electroluminescence display device including a light-emitting layer composed at least of organic material. In the display device, an electrode for a high resistance is electrically connected to a data electrode wire, and an electrode for a low resistance is electrically connected to a scanning electrode wire, to thereby reduce a voltage drop caused by a wire resistance.
Japanese Unexamined Patent Publication No. 10-239655 has suggested a liquid crystal display device including an upper signal line driver circuit and a lower signal line driver circuit between which signal lines extend. The upper and lower signal line driver circuits are electrically connected to each other through first and second power lines. Branch lines extend from the first and second power lines, and are electrically connected to a liquid crystal driver circuit. The first power line is designed such that a half of the first power line extending from a point at which the branch line extend to the liquid crystal driver circuit, to an end of the first power line has a wire resistance equal to a wire resistance of the other half extending from the point to the other end of the first power line. Similarly, the second power line is designed such that a half of the second power line extending from a point at which the branch line extend to the liquid crystal driver circuit, to an end of the second power line has a wire resistance equal to a wire resistance of the other half extending from the point to the other end of the second power line.
In view of the above-mentioned problems in the conventional display devices, it is an object of the present invention to provide a display device which is capable of, even if a bias voltage line would have an increased wire length because of an increase in the number of pixels, reducing and uniformizing a wire resistance of a bias voltage line extending from a bias voltage generating circuit to each of pixels, avoiding reduction in a brightness caused by reduction in a current running through a light-emitting device, resulted from an increase in a wire resistance in a bias voltage line, and avoiding non-uniformity in a brightness in a display screen, caused by non-uniformity in a wire resistance in bias voltage lines extending from a bias voltage generating circuit to each of pixels.
It is also an object of the present invention to provide a display device which is capable of reducing a wire resistance in bias voltage lines to thereby reduce power consumption in the bias voltage lines.
There is provided a display device including (a) a plurality of pixels arranged in a matrix, each of the pixels including a light-emitting device, a switch and a transistor, (b) at least one scanning line extending in a first direction, (c) at least one data line extending in a second direction perpendicular to the first direction, (d) at least one first bias voltage line extending in the second direction, (e) a bias voltage generating circuit which applies a bias voltage to the bias voltage line, (f) a second bias voltage line which surrounds the pixels, and (g) a third bias voltage line which electrically connects the bias voltage generating circuit to the second bias voltage line. The light-emitting device is electrically connected to one of a source and a drain of the transistor. The first bias voltage line is electrically connected to the other of a source and a drain of the transistor. The transistor has a gate electrically connected to the data line through the switch. The first bias voltage line is electrically connected at opposite ends thereof to the second bias voltage line. The switch is turned on when the scanning line is activated, to thereby allow image signals to be transmitted to the gate of the transistor therethrough from the data line. The second and third bias voltage lines are designed to have such a wire resistance that a constant current is supplied to the light-emitting device from the bias voltage generating circuit through the first, second and third bias voltage lines.
It is preferable that the second bias voltage line is rectangular in shape.
The display device may further include a first driver which drives the scanning line a second driver which drives the data line.
The display device may further include a capacitor electrically connected between the gate and the source or drain of the transistor.
It is preferable that the light-emitting device is comprised of an electroluminescence (EL) device.
It is preferable that the second bias voltage line is comprised of a plurality of bias voltage line segments, and that a bias voltage line segment located closer to the bias voltage generating circuit is designed to have a smaller wire resistance per a unit length.
It is preferable that the second bias voltage line is comprised of a plurality of bias voltage line segments, and that a bias voltage line segment located closer to the bias voltage generating circuit is designed to have a broader width.
It is preferable that the bias voltage line segment is tapered in width.
It is preferable that the second bias voltage line is comprised of a first wiring layer having a resistivity smaller than a predetermined resistivity, and a wiring layer of the scanning or data line, the first wiring layer and the wiring layer being vertically layered one on another, the first wiring layer and the wiring layer being connected to each other through a through-hole.
It is preferable that the second bias voltage line has an inner area defined as an area surrounded by itself, the inner area being greater than a predetermined area such that the second bias voltage line acts as a capacitor for removal of noises.
The display device may further include at least one bias bus line extending in the first direction between two portions of the second bias voltage line opposing to each other.
The display device may further include bias bus lines extending in the first direction between two portions of the second bias voltage line opposing to each other, the bias bus lines being arranged by every M pixel rows wherein M is an integer equal to or greater than 1.
The display device may further include bias bus lines extending in the first direction between two portions of the second bias voltage line opposing to each other, the bias bus lines being arranged by every non-constant number of pixel rows.
It is preferable that the second bias voltage line is configured to be a closed loop.
It is preferable that the third bias voltage line has a width greater than a width of said second bias voltage line.
There is further provided a display device including (a) a plurality of pixels arranged in a matrix, each of the pixels including a light-emitting device, a switch and a transistor, (b) at least one scanning line extending in a column direction, (c) at least one data line extending in a row direction, (d) first to N-th first bias voltage lines extending in the column direction wherein N is an integer equal to or greater than 2, (e) a bias voltage generating circuit having first to N-th output terminals through which a bias voltage is applied to the first to N-th first bias voltage lines, (f) first to N-th second bias voltage lines which surround the pixels, and (g) first to N-th third bias voltage lines which electrically connects the first to N-th output terminals of the bias voltage generating circuit to the first to N-th second bias voltage lines, respectively, the light-emitting device being electrically connected to one of a source and a drain of the transistor, the first to N-th first bias voltage lines being electrically connected to the other of a source and a drain of the transistor in the first to N-th rows, the transistor having a gate electrically connected to the data line through the switch, each of the first to N-th first bias voltage lines being electrically connected at opposite ends thereof to an associated second bias voltage line among the first to N-th second bias voltage lines, the switch being turned on when the scanning line is activated, to thereby allow image signals to be transmitted to the gate of the transistor therethrough from the data line, the first to N-th second and third bias voltage lines being designed to have such a wire resistance that a constant current is supplied to the light-emitting device from the bias voltage generating circuit through the first to N-th first, second and third bias voltage lines.
It is preferable that each of the first to N-th second bias voltage lines is rectangular in shape.
The display device may further include a first driver which drives the scanning line a second driver which drives the data line.
It is preferable that each of the first to N-th second bias voltage line is comprised of a plurality of bias voltage line segments, and that a bias voltage line segment located closer to the bias voltage generating circuit is designed to have a smaller wire resistance per a unit length.
It is preferable that each of the first to N-th second bias voltage lines is comprised of a plurality of bias voltage line segments, and that a bias voltage line segment located closer to the bias voltage generating circuit is designed to have a broader width.
It is preferable that the bias voltage line segment is tapered in width.
It is preferable that each of the first to N-th second bias voltage lines is comprised of a first wiring layer having a resistivity smaller than a predetermined resistivity, and a wiring layer of the scanning or data line, the first wiring layer and the wiring layer being vertically layered one on another, the first wiring layer and the wiring layer being connected to each other through a through-hole.
It is preferable that an innermost second bias voltage line among the first to N-th second bias voltage lines has an inner area defined as an area surrounded by itself, the inner area being greater than a predetermined area such that the innermost second bias voltage line acts as a capacitor for removal of noises.
The display device may further include first to N-th bias bus lines each extending in the column direction between two portions of each of the first to N-th second bias voltage lines opposing to each other.
The display device may further include first to N-th bias bus lines each extending in the column direction between two portions of each of the first to N-th second bias voltage lines opposing to each other, each of the first to N-th bias bus lines being arranged by every M pixel rows wherein M is an integer equal to or greater than 1.
The display device may further include first to N-th bias bus lines each extending in the column direction between two portions of each of the first to N-th second bias voltage lines opposing to each other, each of the first to N-th bias bus lines being arranged by every non-constant number of pixel rows.
It is preferable that each of the first to N-th second bias voltage lines is configured to be a closed loop.
It is preferable that each of the first to N-th third bias voltage lines has a width greater than a width of the associated second bias voltage line.
The advantages obtained by the aforementioned present invention will be described hereinbelow.
The display device in accordance with the present invention makes it possible to reduce and uniformize a wire resistance of a bias voltage line extending from a bias voltage generating circuit to each of pixels, even if a bias voltage line would have an increased wire length because of an increase in the number of pixels, avoid reduction in a brightness caused by reduction in a current running through a light-emitting device, resulted from an increase in a wire resistance in a bias voltage line, and avoids non-uniformity in a brightness in a display screen, caused by non-uniformity in a wire resistance in bias voltage lines extending from a bias voltage generating circuit to each of pixels.
The display device in accordance with the present invention also makes it possible to reduce a wire resistance in bias voltage lines to thereby reduce power consumption in the bias voltage lines.
In addition, reduction in power consumption in bias voltage lines ensures an extension in lifetime of bias voltage lines.
Since the second bias voltage line has a large area surrounded by itself, it would be possible to uniformize a bias voltage to be applied through a bias voltage line, and improve an image quality in the display device, by forming a capacitor by means of the second bias voltage line to thereby remove spike noises entering the second bias voltage line.
In addition, it would be possible to optimally compensate for color balance by controlling a bias voltage to thereby control a current running through a light-emitting device, even if a light emission efficiency of the light-emitting device is lowered with an increase in a total period of time during which the light-emitting device emits a light, and resultingly, the light-emitting device is degraded.
The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.